1. Field of the Invention
This invention relates to a method and system for reducing noise in an electromagnetic borehole telemetry system.
2. Description of Prior Art
Electromagnetic telemetry systems are used to transmit information from down in an oil or gas well borehole to equipment located on the surface. A typical borehole telemetry system utilizing electromagnetic means includes a low frequency transmitter located down in the borehole and a signal receiver located on the surface. Electric dipole transmission is already being used as a means of telemetry and magnetic dipole transmission is currently under development. Instead of transmitting electromagnetic signals over conductors in the borehole, the telemetry system transmits the signal through the earth formations surrounding the borehole. One problem associated with these telemetry systems is that of poor signal to noise ratio at the extreme limits of range. Ambient noises include telluric noise and manmade noise from power lines and on-site machinery such as pumps and generators. These noise sources can seriously degrade the usefulness of an electromagnetic telemetry system. Thus, it is desirable to reduce the noise in an electromagnetic telemetry system as much as possible.
The use of auxiliary noise receivers in noise cancellation is not a new idea. The most common embodiment is to use one receiver, far from the signal of interest, to detect magnetotelluric noise and another receiver near the signal source. U.S. Pat. No. 4,980,682 teaches a method of reducing noise in a borehole electromagnetic telemetry system in which one receiver is placed near each noise source. Although the basic method of this patent is basically sound, it has several drawbacks which include (1) the need for a large number of receivers and resulting cabling, (2) the need to identify each noise source, and (3) the use of a complex method for determining the coefficients of each receiver which can involve turning drilling equipment on and off. This is because the weights needed for the receivers are treated as unknowns that need to be determined experimentally.
It is one object oft his invention to provide a method for reducing noise in an electromagnetic borehole telemetry system which overcomes the aforementioned disadvantages.
This and other objects of this invention are addressed by a method for reducing noise in a borehole electromagnetic telemetry system having a signal transmitter disposed in a drill site borehole comprising the steps of positioning at least one signal receiver at a distance from the drill site borehole at which the signal receiver couples strongly to a signal from the signal transmitter and weakly to drill site noise emanating from the drill site borehole and positioning at least one noise receiver at a distance from the drill site borehole at which the noise receiver couples substantially only to magnetotelluric and/or drill site noise. The contribution of the magnetotelluric and/or drill site noise is then determined and subtracted from the signal received by the at least one signal receiver, resulting in a reduced noise signal.
In its simplest form, the method of this invention requires the use of only two receivers disposed on opposite sides of the drill site and oriented to receive a horizontal field component which lies in the line passing through the two receivers and the drill site. The difference in the field at the two receivers is then determined, which difference corresponds to the signal generated by the magnetic signal source. The dipole noise from the drill site does not contribute to this difference and the magnetotelluric noise is canceled while the desired signal is actually increased, as it is the sum from both receivers. Thus, the configuration of this method of the invention reduces both dipole noise from the drill site and magnetotelluric noise using only two receivers.